The present invention relates generally to surface acoustical wave (SAW) devices and more particularly to SAW wave-guides having diffused substrates.
Surface acoustical waveguides in general are formed by confining a channeled (or core) region with a surrounding region (or cladding) having a faster wave velocity, as depicted schematically in FIG. 1. Due to the differences in physical properties, e.g. density and acoustic velocities, the wave will be restricted or channeled in the inner slower region. In FIG. 1, the faster wave propagating regions are shown by cross-hatching, while the slower central region, in which a ray is shown zigzagging back and forth by virture of the total internal reflections at the interface between the slow and fast regions, is shown without cross hatching. Various ways can be utilized to confine the SAW to an internal, channeled region. Materials having different physical properties, such as density, can be utilized so as to achieve a central region with a slower wave propagating velocity effect, or the boundary conditions can be effected to form a channeled region. One of the simplest waveguides developed in the prior art is a thin-film metal overlay that slows down the wave in the guide or channeled region. In piezoelectric crystal substrates, the thin-film metal overlay changes the boundary conditions by creating a short-circuited condition at the interface between the substrate and the overlay. Since the velocity of the wave changes accordingly, the wave is channeled beneath the overlay. SAW devices in which the slow region is created by a metal strip are referred to in the art as .DELTA.v/v waveguides. The .DELTA.v/v refers to the velocity difference between the cladding or surrounding region and the velocity in the core, divided by the velocity with no strip geometry at all. These .DELTA.v/v waveguides have been used in the prior art for amplifiers, long delay lines, and signal processors such as convolvers. In both long delay lines and signal processors some limitations have been experienced due to the effect of phase dispersion, i.e., when different frequencies travel down the guide with unequal phase velocities. Dispersion relations, developed by numerous people, indicate that the effective phase velocity varies between the velocity in the slow region with no waveguides and the velocity in the fast region with no waveguides. The amount of dispersion depends on the size of the velocity changes between the cladding and the core; i.e. a large .DELTA.v/v produces high dispersion. Shown in FIG. 2 is a graphical representation of .DELTA.v/v plotted with respect to the waveguide width "a" multiplied by 2.pi. and divided by .lambda., which is the acoustic wavelength. The three curves represent three different values of .DELTA.v/v (evaluated when a=0). As can be appreciated by those skilled in the art, the dispersion or velocity differences tend to decrease for a lower .DELTA.v/v (evaluated at a=0). The phase dispersion (reflected in FIG. 2 by the slope) is particularly a problem at small wavelengths when power is maximized.
One technique for dispersion compensation in SAW waveguides is mass loading of the core region. This has been suggested by Ronald A. Schmidt and Larry A. Goldren in the publication "Thin Film Acoustic Surface Waveguides on Anisotropic Media" (IEEE Transactions on Sonics and Ultrasonics, Vol. SU-22, No. 2, March, 1975) and some analysis has been performed. According to the analysis, mass loading helps in terms of group dispersion, but aids very little in phase dispersion. Furthermore, mass loading results in energy losses. Another approach has been to compensate externally using a surface wave delay equalizing filter, but that approach requires an additional device and increased circuit insertion losses.
Another type of SAW waveguide is disclosed in U.S. Pat. No. 3,946,338 to Schmidt, as well as in the publication "Acoustic Surface Wave Velocity Perturbations in LiNbO.sub.3 by Diffusion of Metals" (Applied Physics Letters, Vol. 27, No. 1, July 1, 1975), also by R. V. Schmidt. Schmidt disclosed that the diffusion of metals, such as titanium, nickel and chromium into lithium niobate increases the acoustic velocity in the diffused region by a significant amount (up to a value change that is substantially more than one percent) without any readily apparent increase in acoustic loss. Furthermore, Schmidt disclosed a waveguide created by providing regions of uniformly increased acoustic wave velocity about an undiffused channel. Schmidt's device comprises a periodic pattern of grating-like regions produced by diffusing metal into the substrate. However, Schmidt did not consider the use of metal waveguides (overlays) in this configuration.